If you want an accurate Schmitt function, rather than a fast one...

Jan 13, 2022 Last reply: 4 years ago 5 Replies
W

what do you use?



I want to know the high and low switch thresholds, precisely, and for them not to drift.



A '555 relies on its power rails for thresholds, and an op amp needs rail/rail output for the positive feedback to be accurate, neither of which is ideal for a timing component. The old LM322 had a regulator, at least. It's as dead as the XR2206, though.



You can drive the '555 reference, but that's a tad messy.



So, what IS still available that's near ideal, in IC form? Three resistors, a TL431, a dual comparator and an R/S flop would be more messy.


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J

If you are doing timing with an RC, you can charge the cap through a resistor from the same voltage that sets the thresholds. Then the power supply voltage drifts don't matter much.

We use an RC and a dac+comparator to make programmable time delays. If the charging supply is ratiometric on the DAC reference, it's stable.

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S

Available and near ideal (but requires an external reference and $$) LTC1042

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I've used an non-RR op-amp switching a CMOS analog switch, say 1/3 of a 4053. You can use a voltage divider from a reference for the thresholds. Easy-peasy.

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B

Cheep and cheerful:

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P

An RRO comparator such as a TLV7031 and two resistors. Run the op amp off an LP2951 to get decent rails.

Cheers

Phil Hobbs

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P

comparator

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